The present invention pertains generally to automatic gain control circuits used in conjunction with A.C. amplifier circuits and particularly to a fast-attack circuit which may be employed to avoid problems of overattack and pinch-down.
Although automatic gain control (AGC) circuits commonly used in conjunction with A.C. amplifier circuits to compensate for varying input signal levels are capable of tracking minor signal variations to provide a continuous gain control signal, sudden large signal variations require special handling. For example, in order to improve response timme for rapidly attenuating a sudden large signal increase, AGC circuits commonly employ fast-attack circuits for offsetting the delay introduced into the gain control signal by the low-pass filter that is normally required in the gain control loop to minimize harmonic and intermodulation distortion. A typical prior art fast-attack circuit is described in U. S. Pat. No. 3,620,631 entitled "Gain Control Amplifier" which provides an ancillary path for rapidly charging the capacitors in the low-pass filter through a low impedance path in response to signal increases which exceed some predetermined threshold level. Although this circuit and others like it permit quickly stabilizing the AGC control loop (bringing the amplifier circuit output close to its steady state value), during the interim from the inception of the signal increase until the time that the capacitors are sufficiently charged so as to provide an efficacious gain control signal no means are provided for attenuating the output of the amplifier circuit in order to avoid the signal "blast" which results. When this signal "blast" (unattenuated signal) is reflected at the output of the AGC circuit low-pass filter after some finite delay (which is present even with the inclusion of prior art fast-attack circuits), it provides a gain control signal which results in over-attack, viz. a signal which compensates more than necessary so as to reduce the amplifier circuit output to a level that is lower than desired under stablized operation (the output properly attenuated). This lower level is commonly referred to as pinch-down. Although the amplifier output signal will eventually settle back to the desired level, time is expended in doing so during which information may not properly be detected because of the reduced amplitude level.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved fast-attack circuit for use with automatic gain control circuits.
It is a further object of the present invention to provide such a circuit which obviates over-attack problems encountered heretofore and the pinch-down effect associated therewith.